This invention relates to hydraulic cylinder arrangements, such as dashpots, shock absorbers, hydraulic actuators and the like.
In the typical hydraulic actuator or damper a piston slides within a cylinder which is filled with a fluid. When such an arrangement is utilized, as a dashpot or shock absorber, for example, the piston may be fitted with small orifices or passages through which the fluid flows so as to allow the piston to move within the cylinder at a controlled rate determined by the flow rate of the fluid through the orifices. In this way controlled motion or damping of an object connected to the piston through a piston rod is achieved. For reliable operation the piston must be completely immersed in fluid at all times, and the fluid should be substantially free of any air bubbles, since they can alter the flow rate through the orifices and thus produce uneven piston movement and damping action. This is also true for hydraulic actuators because any air in the fluid may decrease the maximum attainable fluid pressurization and thus decrease the actuator power output, mainly because air is compressible, whereas the fluid essentially is not.
In many hydraulic cylinder arrangements the fluid may gradually leak out of the cylinder, especially between the piston rod and the rod seals. The reason is that the seals sustain gradual wear and a small space, through which the fluid may leak, may develop between the seals and the piston rod, as a result. If fluid does leak from the cylinder, the piston may not be completely immersed in fluid throughout its entire range of movement within the cylinder, and uneven operation may result.
One prior art effort to alleviate this problem focuses on the use of two tandem piston rod seals. These are disposed in a chamber to which the fluid is supplied from a separate external reservoir in order to keep the inner seal, which is closest to the piston, well lubricated at all times in order to reduce its wear. Should this inner seal leak, fluid may enter the cylinder from the reservoir on piston depressurization strokes or it may leak from the cylinder, into the reservoir, on pressurization strokes. The outer seal serves to prevent leakage from the reservoir to the outside, but if it leaks, the reservoir will empty ultimately, in which case air will enter the cylinder.
Thus while such approaches are successful for extending cylinder operating life, they do not ameliorate the basic problem of fluid loss. Furthermore, since there is a possibility that the reservoir may empty completely, maintenance is required to ensure that adequate fluid supply is maintained in the reservoir at all times.